Hepatitis B virus (HBV) is a small, partially double-stranded DNA virus that causes acute and chronic hepatitis. An estimated 400 million people are chronically infected worldwide, many suffering early death due to liver failure and primary liver cancer (HCC). The chance of clearing HBV infection is age dependent: approximately 90% of neonatal infections become chronic, whereas at least 90% of adult infections are cleared spontaneously. It is generally accepted that a broad and diverse adaptive immune response is important in clearing acute HBV infection. However, why an individual generates, or fails to generate, a favorable response-and why this capability varies with age-is unknown. The study of HBV immunopathogenesis has been limited because the virus only infects outbred species whose immune systems are difficult to examine, and does not infect mice, the species in which most of the tools to study immunology have been developed. To address this, we have developed the first experimental system to address mechanistic differences in immune responses in very early HBV infection in young and adult humans. This experimental system has produced data that has led to new hypotheses based on experimentation rather than speculation. Prior to these data, the current paradigm posits that immune system immaturity, and neonatal tolerance to HBV underpins the greatly increased viral persistence in the young 6-8, but this has not been mechanistically explained or definitively validated. Our data suggest that immaturity of liver macrophages and monocytes significantly contributes to the immaturity of the immune priming environment of effector cells in the livers of newborns and young children and thus contributes significantly to their inability to generate an effective immune response to HBV. Our proposal seeks to begin to mechanistically understand the cells and signals that are immature, and the cells and signals that are affected by this immaturity. Furthermore, the use of the model outlined in this proposal examines the reversibility of the altered immune priming that facilitates HBV persistence. Finally, the infrastructure is in place to test the conceptual utilityof this model system by confirmation of findings using human liver biopsies. PUBLIC HEALTH RELEVANCE: Hepatitis B virus (HBV) is a small, partially double-stranded DNA virus that causes acute and chronic hepatitis. An estimated 400 million people are chronically infected world- wide, 125 million are people chronically infected in the United States, many suffering early death due to liver failure and primary liver cancer (HCC). There are at least one million deaths annually from HBV-related disease. This makes HBV one of the world's most common and serious human pathogens. The immune response to HBV antigens can result in viral clearance by some individuals, or lead to chronic infection and liver disease in others. The age of the individual at the time of infection dictates the likelihood of HBV clearance. Using a new mouse model of human HBV viral clearance and persistence recently developed in my laboratory, we have recently published a manuscript that demonstrates that interleukin-21 is pivotal in determining age-dependent immune responses. Data in this manuscript also provides evidence that IL-21 may be part of an effective primary hepatic immune response to HBV in patients acutely infected with HBV who clear the virus. Also using this model, we have now uncovered a potential role for liver macrophages and the co-stimulatory molecule OX40 and its ligand OX40L, in age-dependent immune response to HBV and disease outcome. Because these cells, their receptors, and their respective ligands can be manipulated, they are appealing therapeutic targets for this important global pathogen. Our proposed research will shed light on HBV disease pathogenesis and will help identify new potential therapeutic targets to prevent the devastating consequences of chronic HBV infection.